1) Field
Embodiments of the invention are in the field of semiconductor metrology and, in particular, systems and approaches for semiconductor metrology and surface analysis using Secondary Ion Mass Spectrometry (SIMS).
2) Description of Related Art
Secondary ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. The mass/charge ratios of these secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface to a depth of 1 to 2 nanometers. Due to the large variation in ionization probabilities among different materials, SIMS is generally considered to be a qualitative technique, although quantitation is possible with the use of standards. SIMS is the most sensitive surface analysis technique, with elemental detection limits ranging from parts per million to parts per billion.
Generally, SIMS requires a high vacuum with pressures below 10−4 Pa. High vacuum is needed to ensure that secondary ions do not collide with background gases on their way to the detector (i.e., the mean free path of gas molecules within the detector must be large compared to the size of the instrument), and it also prevents surface contamination that otherwise may occur by adsorption of background gas particles during measurement.
Three basic types of ion guns are employed in SIMS analysis. In one, ions of gaseous elements are usually generated with duoplasmatrons or by electron ionization, for instance noble gases (−10Ar, Xe−), oxygen (16O−, 16O2−, 16O2−), or even ionized molecules such as SF5− (generated from SF6) or C60− (fullerene). This type of ion gun is easy to operate and generates roughly focused but high current ion beams. A second source type, the surface ionization source, generates 133Cs− primary ions. Cesium atoms vaporize through a porous tungsten plug and are ionized during evaporation. Depending on the gun design, fine focus or high current can be obtained. A third source type, the liquid metal ion gun (LMIG), operates with metals or metallic alloys, which are liquid at room temperature or slightly above. The liquid metal covers a tungsten tip and emits ions under influence of an intense electric field. While a gallium source is able to operate with elemental gallium, recently developed sources for gold, indium and bismuth use alloys which lower their melting points. The LMIG provides a tightly focused ion beam (less than 50 nanometers) with moderate intensity and is additionally able to generate short pulsed ion beams. It is therefore commonly used in static SLMS devices.
The choice of the ion species and ion gun respectively depends on the required current (pulsed or continuous), the required beam dimensions of the primary ion beam and on the sample which is to be analyzed. Oxygen primary ions are often used to investigate electropositive elements due to an increase of the generation probability of positive secondary ions, while cesium primary ions often are used when electronegative elements are being investigated. For short pulsed ion beams in static SIMS, LMIGs are most often deployed for analysis. LMIGs can be combined with either an oxygen gun or a cesium gun during elemental depth profiling, or with a C60− or gas cluster ion source during molecular depth profiling.
Depending on the SIMS type, there are three basic analyzers available: sector, quadrupole, and time-of-flight. A sector field mass spectrometer uses a combination of an electrostatic analyzer and a magnetic analyzer to separate the secondary ions by their mass to charge ratio. A quadrupole mass analyzer separates the masses by resonant electric fields, which allow only the selected masses to pass through. The time of flight mass analyzer separates the ions in a field-free drift path according to their velocity. Since all ions possess the same kinetic energy, the velocity and therefore time of flight varies according to mass. The time of flight mass analyzer requires pulsed secondary ion generation using either a pulsed primary ion gun or a pulsed secondary ion extraction. The time of flight mass analyzer is the only analyzer type able to detect all generated secondary ions simultaneously, and is the standard analyzer for static SIMS instruments.
In the field of surface analysis, it is usual to distinguish static SIMS and dynamic SIMS. Static SIMS is the process involved in surface atomic monolayer analysis, or surface molecular analysis, usually with a pulsed ion beam and a time of flight mass spectrometer. Meanwhile, dynamic SIMS is the process involved in bulk analysis, closely related to the sputtering process. Dynamic SIMS employs a DC primary ion beans and a magnetic sector or quadrupole mass spectrometer.
SIMS is a very powerful technique. However, advances are needed in the area of SIMS measurement equipment, systems, and methodologies.